CONSTRUCTION OF FUNCTION-SPACES BY MEANS OF THE LAPLACIAN AND THE AVERAGING METHOD Article ZHIDKOV G.V. Differential Equations. Vol. 20. 1984. P. 477-485
VARIATIONAL METHOD FOR SOLVING BOUNDARY-VALUE-PROBLEMS FOR THE WAVE-EQUATION Article FILIPPOV V.M. Differential Equations. Vol. 20. 1984. P. 1401-1407